1. Field of the Invention
The present invention relates to power amplifiers or electrical power supplies. More particularly, the present invention is in the field of power amplifiers for supplying electrical power at a chosen frequency or frequencies and at a selected and variable voltage level or levels. Still more particularly, the present invention is in the field of regulated electronic power amplifiers or supplies which supply a selected level of electrical power at a controlled frequency and variably controlled voltage to an electrical load. With particularity to the application environment of such power amplifiers or power supplies, the present invention relates to such a power amplifier which is particularly configured and constructed for use in the medical environment to supply resistive heating power to a continuous cardiac output monitoring catheter. Thus, the present invention also is in the field of apparatus and method for monitoring the cardiac output of a human patient.
2. Related Technology
Conventionally, cardiac output monitoring for patients experiencing a cardiac crisis, such as may occur over a period of time following a coronary occlusion, is to periodically inject a quantity (or bolus) of chilled saline solution into the patient's circulatory system at a selected location. A temperature monitoring catheter is used at another selected location to sense the temperature-versus-time relationship of the blood flow so that a value of cardiac output can be derived. This technique is known as thermodilution, and provides a good signal to noise ratio of the pulmonary blood flow as it is cooled by the chilled saline solution as compared to the normal temperature of blood flow in the pulmonary artery prior to and after the injection of the bolus of saline. A relationship known as the Stewart-Hamilton equation is used to derive the cardiac output value.
Unfortunately, this conventional technique is dependent on the skill of the person who performs the saline injection. That is, the rate and uniformity over time with which the bolus of saline solution is injected can influence the accuracy of the result. Consequently, a number of such tests over a period of time are used to determine an average value of cardiac output. Detection of a trend or long-term change (over a period of hours, for example) in cardiac output is very difficult with this conventional technique. Also, the injection of chilled saline may have the disadvantage for some patients of adding a relatively large quantity of water to the blood stream. This water must be removed by the patient's kidneys.
Another conventional cardiac output monitoring technique utilizes a catheter instilled through the right atrium and right ventricle of the heart, and from the heart into the pulmonary artery. A resistance heating element externally carried by this catheter is used to intermittently slightly heat the pulmonary blood flow from the heart as this blood flows toward the patient's lungs. Downstream of the heating element, the catheter carries a temperature sensing element. The temperature-versus-time relationship of the sensed blood flow can similarly be used to derive a value for cardiac output. This technique has the advantage of providing substantially continuous monitoring of cardiac output. However, the signal-to-noise ratio of the heated blood temperature in comparison to the normal body temperature of blood flow existing prior to and after an interval of heating is very low. This must be the case because the blood cannot be heated excessively or damage will result to formed blood cells. Consequently, techniques have been developed to heat the pulmonary blood flow on a pseudo-random basis, so that the resulting temperature variations can be detected and distinguished from the otherwise normal slight variations in temperature of the pulmonary blood flow.
For reasons of patient safety and avoidance of electromagnetic interference with or effect upon other monitoring and treatment apparatus which may also be in the medical environment around a patient, a frequency of 100 KHz has been recognized as the most desirable for use in powering the resistance heating element of the monitoring catheter. With this fixed frequency of applied power for the resistance heating element of the catheter, a variable voltage level is used to control the power level of energy liberated at the heating element into the patient's pulmonary blood flow. This control on the level of heating energy liberated into the patient's blood flow must be carefully controlled because the actual rate of blood flow circulation for the patient may be decreased or impaired, so that overheating must be avoided.
In addition to the above, it is increasingly recognized that the modern medical environment is restrictively complex. That is, the complexity of medical monitoring and treatment apparatus which must be used with some seriously ill or injured patients restricts access to the patient and presents the risk of error or malfunction of the apparatus. Additionally, hospitals and clinics face a significant burden in maintenance, service, storage, and logistical planning of the availability of this complex and expensive medical apparatus. As a result, an increasingly popular trend in the hospital, clinical, and portable medical treatment environments (fire departments, emergency medical teams, and military portable field hospitals, for example) is to use a general purpose monitoring device which can be electronically configured to serve a variety of monitoring functions.
Configuration of the monitoring device is accomplished by simply plugging into the console of the general purpose monitoring device one or more modules containing the circuitry and stored information necessary to accomplish particular monitoring functions. In the hospital and clinical environment, for example, this technology has the advantage that the general purpose monitors may be installed in or left in the patient rooms and in the emergency or critical care areas, for example. These monitors need not be moved about the hospital or clinic. The monitors are simply configured to perform various monitoring functions as are necessitated by the condition of the particular patient by plugging the appropriate modules into the monitor consoles. Only the modules need to be moved about the hospital or clinic. The modules themselves are comparatively small, light and inexpensive. Storage of the modules when they are not in use requires far less space than does the conventional monitoring equipment. Also, movement of the necessary configuration modules about the hospital or clinic environment does not present nearly the burden for hospital staff as does the movements of conventional monitors.
That is, conventional monitors are relatively large, heavy, and expensive pieces of equipment, which are generally mounted on wheeled carts. Each time a monitor is moved from one location to another within a hospital, for example, there is a certain risk that it will be damaged in the process of movement. Also, the physical movement of the monitor requires the services of a relatively strong member of the hospital service personnel, for example, to move the wheeled cart and monitor onto and off of hospital elevators. On the other hand, the configuration modules of modular type monitoring equipment are small enough to be carried by hand from one location to another. In fact, several of these modules can be carried at a time by one person if necessary. A single wheeled cart of a size comparable to one conventional monitor can carry several to several dozen of the configuration modules for a modular monitoring system.
With respect to the conventional continuous cardiac output monitors, the monitor includes a linear electronic power amplifier capable of supplying a variable power level and fixed frequency of electrical alternating current power output, and which provides electrical heating power to the resistance heating element of the continuous cardiac output monitoring catheter. This conventional linear power amplifier is physically too large to be accommodated within the envelope of a monitoring module of the newer modular-type of monitoring apparatus. Also, the conventional power amplifier is of a power efficiency so low that although only about fifteen watts of power is dissipated into the patient's blood flow on an intermittent basis, about thirty to forty-five watts, or more, of power is liberated as heat into the console of the conventional continuous cardiac output monitor. That is, the efficiency of these conventional linear power amplifiers may be as low as 25 percent. Were this level of heat to be liberated within a monitoring module, assuming that the conventional linear power amplifier could somehow be physically fitted into the module, the conventional plastic casing of the module could be warped or melted by the resulting high temperatures.